There's nothing unusual about multiple signatures on a treaty. For the various parties to the Treaty of Versailles, for example, there were between 1 and 5 signatures each.

For the Rome Treaty, signing the treaty was not the legally consequential step (or most treaties). To actually go into force, the treaty required ratification by each of the parties according to the appropriate domestic procedure. States often fail to ratify treaties, and there's not really a need for a fail safe at the signature phase given ratification. Under international law, states are bound to a treaty by whatever means they agree to express consent.

The actual signing of a treaty, therefore, is a basically ceremonial act and giving your signature is an honor. Interestingly enough, and underlining the ceremonial nature of the action, the Treaty of Rome as signed was actually blank through a comedy of administrative errors.